熊本県肥後変成帯に産する深成岩類の形成年代と冷却年代

Abstract

Higo plutonic rocks as a part of the Higo metamorphic terrane occur in the central part of Kyushu island, southwest Japan. They are divided mainly into two types, the Miyanohara tonalite and the Shiraishino granodiorite. The Shiraishino granodiorite intrudes into both of the Miyanohara tonalite and the Higo metamorphic rocks. The inclination line for the nine samples from the Miyanohara tonalite gives an age of 121±65 Ma and is not defined as solidification age of this tonalite because of a large error. A Rb-Sr whole rock isochron for the Shiraishino granodiorite indicates an age of 121±14 Ma with an initial ratio of 0.704929±0.000070 (2σ). This whole rock isochron age is regarded as the solidification age for the Shiraishino granodiorite. A Rb-Sr mineral isochron is formed by whole-rock, hornblende, K-feldspar and plagioclase from one rock sample for the Shiraishino granodiorite. This mineral isochron age indicates an age of 100.0±1.7 Ma, and is regarded as the cooling age for the Shiraishino granodiorite. The Rb-Sr whole rock age and Sr initial ratio for the Shiraishino granodiorite are within the range of those of the Cretaceous granitic rocks that occur in the central Kyushu island. Since the timing of peak metamorphism of the Higo metamorphic rocks is believed to be c. 260-240 Ma from their geochronological view points, the magmatism for the Shiraishino granodiorite would not relate with the metamorphism of the Higo metamorphic rocks.